Two-time champion Venus Williams is out of the U.S. Open following a battling 6-3 2-6 7-6 (7-5) second-round defeat to world No. 56 Zheng Jie.

Former world No. 1 Williams, currently ranked 60th, fought bravely against her Chinese opponent before eventually losing in three hours and two minutes.

The marathon encounter was the joint fifth-longest women's singles match in the tournament's history.

In recent years Williams has struggled with form and fitness and entered the New York tournament with a back injury. She was diagnosed with the auto-immune disorder Sjogren's Syndrome at Flushing Meadows two years ago.

"I think there's a lot of positives for me out of this tournament. I continue to serve better and recover [from] my back injury with each tournament," the 33-year-old told the event's official website.